A fuel cell, which is a new generation system using electrical energy directly converted from the energy produced by the electrochemical reaction of a fuel gas and an oxidant gas, is under careful examination for use as power generation equipment, such as that for space stations, unmanned facilities at sea or along coastal areas, fixed or mobile radios, automobiles, household electrical appliances or portable electrical appliances.
Fuel cells are divided into a molten carbonate electrolyte fuel cell operated at a high temperature (in the range from about 500.degree. C. to about 700.degree. C.), a phosphoric acid electrolyte fuel cell operated around 200.degree. C., an alkaline electrolyte fuel cell operated at a room temperature to about 100.degree. C., and a solid electrolyte fuel cell operated at a very high temperature (1,000.degree. C. or above).
A molten carbonate fuel cell (MCFC) is constituted by a porous Ni anode, Li-doped porous Ni oxide cathode and lithium aluminate matrix which is filled with lithium and potassium carbonate as electrolytes. The electrolytes are ionized by melting at about 500.degree. C. to 700.degree. C., which is the operating temperature of the cell, and the carbonate ions generated therefrom transport charges between the electrodes. Hydrogen is consumed in the anode area producing water, carbon dioxide and electrons. The electrons flow to the cathode via an external circuit to produce the desired current flow.
Porous anodes of nickel, cobalt or copper base are typically used in MCFCs. As mentioned above, MCFCs are operated at a high temperature of about 650.degree. C., and pressure is also applied thereon in order to improve the contact between electrodes and an electrolyte matrix. Various layers are stacked to produce pressure by the loading of the stack, so that a creep deformation which deforms the structure of the anode occurs. Creep deformation of electrodes occurs by a combination of at least the three different mechanisms of particle rearrangement, sintering and dislocation movement. Due to non-uniform creeps of the respective parts of the anode, the minute pores are reduced, thereby reducing the reactive area of the electrode, creating a poor contact between electrodes and electrolyte elements and leading to the possibility of fuel gas leakage, so that electrode performance is lowered in various respects.
Therefore, various methods has been attempted to prevent such an undesirable creep deformation in MCFC anodes. One method is to manufacture the anode by adding Cr or Al to Ni. For example, when a creep test is performed for 100 hours at the conditions of 100 psi and 650.degree. C., the pure Ni electrode has about 30% to 50% creep. Ni--LiAlO.sub.2, which is made by adding LiAlO.sub.2 to Ni electrode, has about 14% to 35% creep. Likewise, in the case of Ni--Cr (10%) to which 10% Cr is added and Ni--Al (10%) to which 10% Al is added, the creeps are lowered to 5% to 10% and to about 2%, respectively. In operating MCFCs of a large capacity, however, a satisfactory performance for a long time (40,000 hours) has not yet been achieved.
Although Ni--Cr (10%) anodes have been widely used so far, since the price of Cr is high, the addition of Al is considered. If Al is added to Ni, the creep deformation is lowered to 2% or less. Besides, Al is cheaper than Cr.
A Ni--Al anode or Ni--(Al,Cr) anode made by adding Al or Cr to Ni is manufactured by the same procedure as that used in the art by forming a green sheet by a casting method after forming an alloy powder of Ni and additive metals. However, it is difficult to form a minute alloy powder of Ni and metals.
One method for preventing creep deformation of a porous anode structure is to internally oxidize alloy metals typically used in the base metal-alloy metal composition.
For example, U.S. Pat. No. 4,315,777 discloses a method for internally oxidizing alloy metal by a heat treatment of the powder blend of an alloy powder and an oxidant base metal, but it is not suitable for use as a porous anode structure because of the high density of the end product.
U.S. Pat. No. 4,714,586, which discloses a method for forming dimensionally stable Ni--Cr anodes by internally oxidizing the alloy metal at high water vapor pressures, is limited to the formation of Ni--Cr anodes.
U.S. Pat. No. 3,578,443 discloses a method for producing an oxide dispersion strengthened alloy and then sintering by surface oxidizing Cu, Al powder by contact with an alcohol suspension and internally oxidizing all the powder by heat treatment in an airtight tube at 750.degree. C. The material produced by this method is not suitable as a porous anode structure because of its high density.
To solve the aforementioned problems, U.S. Pat. No. 4,999,155 discloses a method for forming MCFC anodes with an improved creep resistance property. That is to say, a base metal powder and alloy metal powder are blended with a binder and solvent, then the blend is dried, sintered, and cast to form a porous anode structure. The alloy metal is internally oxidized under conditions in which the base metal is reduced and the alloy metal is oxidized to form oxide particles therein.
Another method disclosed in the above patent is forming an alloy including a base metal and an alloy metal, oxidizing the surface of the alloy by a heat treatment, and then simultaneously sintering and internally oxidizing the oxidized alloy. Here, the introduction of the alloy metal to the base metal is achieved by tape casting and sintering the mixture of the base metal powder and the alloy metal powder and then diffusing the alloy metal to the base metal. Alternatively, a vapor deposition or a pack cementation is performed after the base metal powder is tape cast and sintered into a porous structure.
According to the above method, Ni metal powder is first mixed with binder and solvent, then tape cast, dried and sintered to form a porous Ni sintered body. Then, the porous nickel sintered body is embedded in a pack consisting of alloy metal powder such as Al, an activator salt and an inert filler, and is heated to about 900.degree. C. in an atmosphere of 10%-H.sub.2 /90%-N.sub.2, performing a pack cementation and thereby forming Ni--Al alloy. Next, the structure is then internally oxidized at temperatures of 600.degree. C. to 800.degree. C. in a humidified atmosphere having a pH.sub.2 O/pH.sub.2 ratio greater than 20. Although the method has an excellent effect in decreasing creep deformation, since various complicated procedures such as drying, sintering, pack cementation and internal oxidization should be performed after forming a green sheet with a Ni base metal, this method lacks utility.